Neuronal replacement shows promise for enhancing recovery after spinal cord injury (SCI), and is considered a major objective for stem cell-mediated spinal cord repair. Achieving effective host-graft neuronal communication, however, represents a major challenge for neuronal replacement which has not been investigated. The overall hypothesis of this proposal is that presenting newly established host-graft neuronal networks with physiologically-patterned activities will enhance functional connectivity. Based on our extensive experience with SCI modeling and both respiratory and computational neurobiology, we are proposing to test this hypothesis by transplanting rat fetal spinal cord (FSC) tissue grafts - a source of neuronal progenitors - into clinically-relevant, high cervical (C3) contusion injuries in adult rats. Preliminary transneuronal neuroanatomical tracing data show synaptic connectivity between FSC donor cells and host phrenic motoneurons and cervical interneurons. In addition, FSC grafts receive extensive serotonergic inputs from the host, and some graft neurons exhibit hypoxia-sensitive discharge patterns including apparent inspiratory- related bursting. A prerequisite anatomical-functional framework is thus in place to test our central hypothesis via the following specific aims using FSC grafts placed into C3 contusion injuries in adult rats: Aim 1) to test the hypothesis that FSC-derived neurons will become anatomically and physiologically integrated with host gray matter, and Aim 2) to test the hypothesis that "training" via intermittent hypoxia (IH) stimulation will enhance the anatomical and physiological integration of FSC-derived neurons. To test these hypotheses, we will use a multi-disciplinary approach including neuroanatomical studies of connectivity between the graft and host spinal cord, measurement of breathing in awake rats, and neurophysiological studies of the graft. An innovative technical feature of our proposal is that this will be the first use of microelectrode arrays to monitor graft- associated neural ensembles in the spinal cord. A FSC grafting method will be used because such grafts develop into myelinated tissue containing a large contingent of cells resembling intermediate gray matter interneurons. A unique rehabilitative paradigm - daily exposure to mild, intermittent hypoxia (IH) - will be used because it leads to spinal cord plasticity associated with persistent increases in respiratory output in spinal injured animals. Equally important, IH provides a tool to introduce or increase appropriately patterned bursting around, and possibly within, the graft. This proposal brings together unique expertise in respiratory neurophysiology, computational neurobiology, neural transplantation, and SCI in an effort to facilitate transformative advances in the understanding and treatment of SCI. PUBLIC HEALTH RELEVANCE: Respiratory compromise is a significant problem after cervical spinal cord injury. A strategy that may enhance motor recovery after spinal injury is "neural replacement" therapy in which cells are transplanted into the spinal cord lesion. In these experiments, we will examine if a novel rehabilitation paradigm can enhance the effectiveness of a neural transplant following spinal cord injury.